Selective etching bath methods

ABSTRACT

An etching method. The method includes etching a first plurality of silicon wafers in a first enchant, each silicon wafer having SiO 2  and Si 3 N 4  deposited thereon, where the etching includes dissolving a quantity of the SiO 2  and a quantity of the Si 3 N 4  in the first echant. A quantity of insoluble SiO 2  precipitates. A ratio of a first etch rate of Si 3 N 4  to a first etch rate of SiO 2  is determined to be less than a predetermined threshold. A portion of the first etchant is combined with a second etchant to form a conditioned etchant. A second plurality of silicon wafers is etched in the conditioned etchant. A ratio of a second etch rate of Si 3 N 4  to a second etch rate of SiO 2  in the conditioned etchant is greater than the threshold. A method for exchanging an etching bath solution and a method for forming a selective etchant are also disclosed.

FIELD OF THE INVENTION

The invention relates to semiconductor etching bath methods, materials,and systems.

BACKGROUND OF THE INVENTION

Phosphoric acid may be used as an etchant in semiconductor processing toselectively etch silicon nitride over silica from silicon processwafers. A build up of dissolved silica in the phosphoric acid etch bathfrom the etching of wafers, may inhibit further etching of silica fromthe wafer and thus increase the selectivity of the etchant. However, asthe silica content of the etchant bath reaches saturationconcentrations, the build up of solid silica precipitate may require thecomplete exchange of etchant from the bath system to avoid damagingpumps, filters, and other components. Replacement of the phosphoric acidin the bath with fresh acid requires a “seasoning” period to bring thesilica concentration in solution back up to levels associated with highselectivity, where during this “seasoning” period the etch bath has lowselectivity and thus low efficiency for etching silicon nitride. The useof purchased “preseasoned” acid solutions is costly in terms of bothmaterials and the increased complexity of separate handling equipment.There exists a need for a cost effective method for replenishing etchingbaths with minimal impact on process complexity.

SUMMARY OF THE INVENTION

The present invention relates to an etching method, comprising:

etching a first plurality of silicon wafers in a first enchant, eachsilicon wafer of said first plurality of silicon wafers having SiO₂ andSi₃N₄ deposited thereon, wherein said etching comprises dissolving aquantity of said SiO₂ and a quantity of said Si₃N₄ in said first echantand a quantity of insoluble SiO₂ precipitates in said first echant inresponse to said dissolving;

after said etching, determining that a ratio of a first etch rate ofSi₃N₄ to a first etch rate of SiO₂ from said first plurality of siliconwafers in said first etchant is less than a predetermined threshold;

in response to said determining and after said etching, combining aportion of said first etchant with a second etchant to form aconditioned etchant, wherein said portion is essentially free ofinsoluble SiO₂; and

after said combining, etching in said conditioned etchant a secondplurality of silicon wafers having SiO₂ and Si₃N₄ disposed thereon,wherein a ratio of a second etch rate of Si₃N₄ to a second etch rate ofSiO₂ from said second plurality of silicon wafers in said conditionedetchant is greater than said predetermined threshold.

The present invention relates to a method for forming a selectiveetchant, comprising:

etching a quantity of SiO₂ and a quantity of Si₃N₄ from a substrate in afirst etchant, said etching comprising dissolving said quantity of SiO₂and said quantity of Si₃N₄ in said first etchant;

determining a ratio of an etch rate of Si₃N₄ to an etch rate of SiO₂from said substrate in said first etchant, based on said etching;

determining that said ratio is less than a predetermined threshold; and

in response to said determining that said ratio is less than saidpredetermined threshold, combining a second etchant with a portion ofsaid first etchant to form a selective etchant, said first etchanthaving said quantity of SiO₂ and said quantity of Si₃N₄ dissolvedtherein, wherein said portion is in a sufficient amount such that saidselective etchant selectively etches Si₃N₄ over SiO₂ in a ratio greaterthan said predetermine threshold.

The present invention relates to a method for exchanging an etching bathsolution, comprising:

providing an etching system comprising an etching bath having a firstetchant disposed therein, a receiving tank connected to said etchingbath and configured to collect said first etchant transferred from saidbath;

etching Si₃N₄ and SiO₂ from at least one substrate into said firstetchant, said etching comprising dissolving said SiO₂ and said Si₃N₄ insaid first etchant;

determining that an etching rate ratio of Si₃N₄ to SiO₂ from said atleast one substrate in said first etchant is below a threshold value;

in response to said determining, transferring said first etchant fromsaid bath to said receiving tank;

after said transferring said first etchant, adding a second etchant tosaid bath; and

transferring a portion of said first etchant from said receiving tank tosaid bath, said portion mixing with said second etchant in said bath,said first etchant having said Si₃N₄ and said SiO₂ dissolved therein.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of illustrative embodiments when read inconjunction with the accompanying drawings.

FIG. 1 is a flow chart illustrating a method for forming a selectiveetchant, in accordance with embodiments of the present invention.

FIG. 2 is a flow chart illustrating an example of an embodiment of anetching method, in accordance with embodiments of the present invention.

FIG. 3 is an illustration of an etching system comprising an etchingbath having an etchant disposed therein, in accordance with embodimentsof the present invention.

FIG. 4 is a flow chart illustrating an example of a method forexchanging an etching bath solution, in accordance with embodiments ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Although certain embodiments of the present invention will be shown anddescribed in detail, it should be understood that various changes andmodifications may be made without departing from the scope of theappended claims. The scope of the present invention will in no way belimited to the number of constituting components, the materials thereof,the shapes thereof, the relative arrangement thereof, etc., and aredisclosed simply as examples of embodiments. The features and advantagesof the present invention are illustrated in detail in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout the drawings. Although the drawings are intended toillustrate the present invention, the drawings are not necessarily drawnto scale.

FIG. 1 is a flow chart illustrating a method for forming a selectiveetchant. Step 200 comprises etching a quantity of silica (SiO₂) and aquantity of silicon nitride (Si₃N₄) from a substrate in a first etchantwhere the etching comprises dissolving the quantity of silica and thequantity of silicon nitride in the first etchant.

The etchants in embodiments described herein may comprise a liquidetchant, such as a liquid acidic etchant, such as concentratedphosphoric acid, where the concentration of acid in the etchant may begreater than about 85% by weight, such as between about 85% and about95%. The etchant may be heated such that the etchant is at a temperaturebetween about 100° C. to about 165° C. during the etching in step 200.

The substrate may have silica and silicon nitride disposed thereon, suchas in the form of layers, for example. The substrate may comprisemultiple layers or the substrate may comprise a single layer. Thesubstrate may consist essentially of silica and silicon nitride. Thesubstrate may include a semiconducting material, an insulating material,a conductive material or any combination thereof, including multilayeredstructures. Thus, for example, the substrate may comprise asemiconducting material such as Si, SiGe, SiGeC, SiC, GaAs, InAs, InPand other III/V or II/VI compound semiconductors. The substrate maycomprise, for example, a silicon wafer or process wafer such as thatproduced in various steps of a semiconductor manufacturing process, suchas an integrated semiconductor wafer. The substrate may comprise alayered substrate such as, for example, Si/SiGe, Si/SiC,silicon-on-insulators (SOIs) or silicon germanium-on-insulators (SGOIs).The substrate may comprise layers such as a dielectric layer, a barrierlayer for copper such as SiC, a metal layer such as copper, a siliconlayer, a silicon oxide layer, the like, or combinations thereof. Thesubstrate may comprise an insulating material such as an organicinsulator, an inorganic insulator or a combination thereof includingmultilayers. The substrate may comprise a conductive material, forexample, polycrystalline silicon (polySi), an elemental metal, alloys ofelemental metals, a metal silicide, a metal nitride, or combinationsthereof, including multilayers. The substrate may comprise ion implantedareas, such as ion implanted source/drain areas having P-type or N-typediffusions active to the surface of the substrate.

In some embodiments, the substrate may include a combination of asemiconducting material and an insulating material, a combination of asemiconducting material and a conductive material or a combination of asemiconducting material, an insulating material and a conductivematerial. An example of a substrate that includes a combination of theabove is an interconnect structure.

Step 205 comprises determining a ratio of an etch rate of Si₃N₄ to anetch rate of SiO₂ from the substrate in the first etchant, based on theetching performed in step 200. The etch rate of Si₃N₄ and the etch rateof SiO₂ from the substrate may be determined experimentally, forexample, by measuring the time in the etchant and measuring thethickness change in Si₃N₄ and SiO₂ on the substrate and determining thethickness change as a function of time. A high etching selectivity forSi₃N₄ over SiO₂ is indicated by a high etch rate ratio of Si₃N₄ to SiO₂.For example, a high selectivity may be indicated by an etch rate ratiogreater than about 50:1, such as when an etch rate of SiO₂ is less thanabout 1 angstrom/minute (Å/min) and an etch rate of Si₃N₄ is greaterthan about 50 Å/min. In one embodiment, the etch rate ratio of Si₃N₄ toSiO₂ may be in a range from about 50:1 to about 200:1.

The etch rate ratio may initially be low when first etching thesubstrate in an etchant essentially free of SiO₂. For example, whenetching a substrate having Si₃N₄ and SiO₂ disposed on the substrate,where the etchant is concentrated phosphoric acid such as insemiconductor processing, initially the etch rate ratio may be low (suchas below 50:1) until a sufficient concentration of SiO₂ dissolves intothe etchant solution bringing the concentration of SiO₂ in solution toabout saturation, thus inhibiting the dissolving of SiO₂ into solutionand therefore inhibiting etching of SiO₂ in the etchant. This may bedescribed as “seasoning” the first etchant, after which the etch rateratio may stabilize between about 50:1 to about 200:1. For example, asufficient concentration may be attained at approximately 400wafer-minutes of etching, where a wafer-minute may be described as onewafer etched for one minute in the etchant.

Step 210 comprises determining that the ratio determined in step 205 isless than a predetermined threshold. The determining of step 210 may beperformed when the etch rate ratio has first stabilized, as describedabove, and then drops below the threshold as more SiO₂ dissolves in theetchant. The predetermined threshold may be about 50:1, for example. Adecrease in the ratio to a value below the predetermined threshold mayindicate an increasing concentration of Si₃N₄ and/or SiO₂ dissolved inthe etchant, where high concentrations of SiO₂ may cause precipitationof solid SiO₂.

Step 215, in response to determining in step 210 that the ratio is lessthan the predetermined threshold, combines a second etchant with aportion of the first etchant to form a selective etchant, said firstetchant having said quantity of SiO₂ and said quantity of Si₃N₄dissolved therein, wherein said portion is in a sufficient amount suchthat said selective etchant selectively etches Si₃N₄ over SiO₂ in aratio greater than said predetermine threshold. The selective etchantmay comprise between about 1% and about 30% of the first etchant byvolume. The addition of the portion of the first etchant into the secondetchant may “preseason” the second etchant and stabilize the etch rateratio above about 50:1 in the resulting selective etchant, since addingthe portion adds a dissolved quantity of Si₃N₄ and SiO₂.

The first etchant and the second etchant may each comprise a liquidacidic etchant, such as phosphoric acid. The second etchant may beessentially free of SiO₂ before adding the portion of the first etchantto the second etchant.

FIG. 2 is a flow chart illustrating an example of an embodiment of anetching method. Step 250 includes etching a first plurality of siliconwafers in a first etchant. Each silicon wafer of said first plurality ofsilicon wafers may have SiO₂ and Si₃N₄ deposited thereon. The etchingmay comprise dissolving a quantity of the SiO₂ and a quantity of theSi₃N₄ in the first echant from each of the plurality of wafers. Aquantity of insoluble SiO₂ may precipitate in the first echant inresponse to dissolving the SiO₂ in the etchant.

For example, the plurality of silicon wafers may be siliconsemiconductor process wafers having at least one layer of Si₃N₄ and atleast one layer of SiO₂ disposed on the wafer. The plurality of wafersmay be etched in the etchant which may be concentrated phosphoric acid,for example. A sufficient quantity of wafers for a sufficient quantityof time may etch enough SiO₂ such that SiO₂ begins to precipitate out ofsolution. For example, etching for greater than 20,000 wafer-minutes mayproduce SiO₂ precipitate in the etchant.

Step 255 comprises determining, after the etching of step 250, that aratio of a first etch rate of Si₃N₄ to a first etch rate of SiO₂ fromsaid first plurality of silicon wafers in said first etchant is lessthan a predetermined threshold. As described above for the steps of FIG.1, the etch rate ratio for Si₃N₄ and SiO₂ may decrease as SiO₂ and Si₃N₄are dissolved in the first etchant, where the etch rate ratio may bedetermined experimentally from the etching of the plurality of siliconwafers. The ratio may drop below a predetermined limit or threshold,such as below 50:1, where the selectivity may be too low to continueprocessing additional silicon wafers at a desired level of efficiency.For example, the etch rate of Si₃N₄ may be less than about 50 Å/min.and/or an etch rate of SiO₂ may be greater than about 1 Å/min.

Step 260 comprises, in response to the determining of step 255 and afterthe etching of step 250, combining a portion of the first etchant with asecond etchant to form a conditioned etchant. The conditioned etchantmay comprise between about 1% and about 30% of the first etchant. Theportion of the first etchant may be essentially free of solid SiO₂. Forexample, the portion of the first etchant may be siphoned off from thebulk of the first etchant such that precipitated (solid) SiO₂ is notextracted into the portion of the first etchant. The portion of thefirst etchant may thus comprise dissolved SiO₂ and Si₃N₄ withoutinsoluble SiO₂. Combining the portion of the first etchant havingdissolved SiO₂ and Si₃N₄ with a second etchant may immediately form aconditioned etchant capable of etching silicon wafers at an etch rateratio above the predetermined threshold.

Step 265 comprises, after the combining of step 260, etching in theconditioned etchant a second plurality of silicon wafers having SiO₂ andSi₃N₄ disposed thereon, wherein a ratio of a second etch rate of Si₃N₄to a second etch rate of SiO₂ from said second plurality of siliconwafers in said conditioned etchant is greater than said predeterminedthreshold. The first etchant and the second may each comprise a liquidacidic etchant, such as phosphoric acid. The second etchant may beessentially free of SiO₂ before adding the portion of the first etchantto the second etchant. For example, for a silicon semiconductor etchingprocess, conditioned phosphoric acid etchant may be used for continuedetching of wafers after combining the portion of the first phosphoricacid having SiO₂ and Si₃N₄ dissolved therein, with a fresh amount ofphosphoric acid essentially free of SiO₂, to form the conditionedetchant.

FIG. 3 is an illustration of an etching system 100 comprising an etchingbath 105 having an etchant 110 disposed therein. The etching system 100may comprise a receiving tank 120 connected to the etching bath 105,where the receiving tank 120 may be configured to collect drainedetchant 125 such as the etchant 110 which may be drained or transferredfrom the bath 105. The etching system 100 may comprise at least one pump115 operably attached to the receiving tank 120 and to the etching bath105. The at least one pump 115 may be configured to transfer the etchant125 from the receiving tank 120 to the bath 105, and may be configuredto pump the etchant 110 from the bath 105 to the receiving tank 120 suchas when the bath 105 is being drained. The etching system may furthercomprise at least one valve 135 configured to direct the flow of etchant110, such as while it is drained from the bath 105, or as it iscirculated through at least one filter 130 and returned to the bath 105.The etching system 100 may comprise a drain 140 through which etchantmay be removed and disposed of.

FIG. 4 is a flow chart illustrating an example of a method forexchanging an etching bath solution. Step 400 comprises providing anetching system comprising an etching bath having a first etchantdisposed therein, a receiving tank connected to said etching bath andconfigured to collect said first etchant transferred from said bath. Forexample, the etching system may comprise the etching system 100illustrated in FIG. 3 and described above. The first etchant maycomprise an acidic etchant, such as concentrated phosphoric acid. Thefirst etchant may be at a temperature between about 100° C. and about165° C. in the bath.

Step 405 comprises etching Si₃N₄ and SiO₂ from at least one substrateinto said first etchant, said etching comprising dissolving said SiO₂and said Si₃N₄ in said first etchant. The substrate may be as describedabove where, for example, the substrate may be a plurality of siliconprocess wafers.

Step 410 comprises determining that an etching rate ratio of Si₃N₄ toSiO₂ from said at least one substrate in said first etchant is below athreshold value, where the etching rate ratio is the ratio of theetching rate of Si₃N₄ to the etching rate of SiO₂ in the first etchant.Determining such an etch rate ratio may be as described above where, forexample, the threshold value may be about 50:1, where the Si₃N₄ etchedfrom the substrate in step 405 may be etched at an etch rate below about50 Å/min, where the SiO₂ etched from the substrate in step 405 may beetched at an etch rate above about 1 Å/min.

Step 415 comprises, in response to determining of step 410, transferringthe first etchant from the bath to the receiving tank. The receivingtank may be configured to cool or heat the drained etchant in order tochange the etchant's temperature to a previously determined temperature.For example, for a phosphoric acid etchant in a bath maintained in atemperature range between about 100° C. and about 165° C., the drainedetchant may be cooled to below 100° C. such as ambient temperature, ormay be maintained within the temperature range of the bath.

Step 420 adds a second etchant to the bath after transferring the firstetchant to the receiving tank in step 415. The second etchant maycomprise an acidic etchant such as concentrated phosphoric acid and maybe essentially free of SiO₂.

Step 420 transfers a portion of the first etchant having the Si₃N₄ andthe SiO₂ dissolved therein, from the receiving tank to the bath, wherethe portion mixes with the second etchant added to the bath in step 420.The transferring may comprise pumping the portion of the first etchantusing the pump described above and illustrated in FIG. 3. Mixing theportion of the first etchant with the second etchant in the bath mayform a final volume of etchant, where the portion of the first etchantis between about 1% and about 30% of the final volume of etchant.

The foregoing description of the embodiments of this invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to aperson skilled in the art are intended to be included within the scopeof this invention as defined by the accompanying claims.

1. An etching method, comprising: etching a first plurality of siliconwafers in a first enchant, each silicon wafer of said first plurality ofsilicon wafers having SiO₂ and Si₃N₄ deposited thereon, wherein saidetching comprises dissolving a quantity of said SiO₂ and a quantity ofsaid Si₃N₄ in said first echant and a quantity of insoluble SiO₂precipitates in said first echant in response to said dissolving; aftersaid etching, determining that a ratio of a first etch rate of Si₃N₄ toa first etch rate of SiO₂ from said first plurality of silicon wafers insaid first etchant is less than a predetermined threshold; in responseto said determining and after said etching, combining a portion of saidfirst etchant with a second etchant to form a conditioned etchant,wherein said portion is essentially free of insoluble SiO₂; and aftersaid combining, etching in said conditioned etchant a second pluralityof silicon wafers having SiO₂ and Si₃N₄ disposed thereon, wherein aratio of a second etch rate of Si₃N₄ to a second etch rate of SiO₂ fromsaid second plurality of silicon wafers in said conditioned etchant isgreater than said predetermined threshold.
 2. The method of claim 1,wherein said second etchant is essentially free of SiO₂.
 3. The methodof claim 1, wherein said first etchant and said second etchant eachcomprise phosphoric acid.
 4. The method of claim 1, wherein saidpredetermined threshold is about 50:1.
 5. The method of claim 4, whereinsaid first etch rate of Si₃N₄ is less than about 50 angstroms/minute. 6.The method of claim 4, wherein said first etch rate of said SiO₂ isgreater than about 1 angstrom/minute.
 7. The method of claim 1, whereinsaid conditioned etchant comprises between about 1% and about 30% ofsaid first etchant.
 8. A method for forming a selective etchant,comprising: etching a quantity of SiO₂ and a quantity of Si₃N₄ from asubstrate in a first etchant, said etching comprising dissolving saidquantity of SiO₂ and said quantity of Si₃N₄ in said first etchant;determining a ratio of an etch rate of Si₃N₄ to an etch rate of SiO₂from said substrate in said first etchant, based on said etching;determining that said ratio is less than a predetermined threshold; andin response to said determining that said ratio is less than saidpredetermined threshold, combining a second etchant with a portion ofsaid first etchant to form a selective etchant, said first etchanthaving said quantity of SiO₂ and said quantity of Si₃N₄ dissolvedtherein, wherein said portion is in a sufficient amount such that saidselective etchant selectively etches Si₃N₄ over SiO₂ in a ratio greaterthan said predetermine threshold.
 9. The method of claim 8, wherein saidsecond etchant is essentially free of SiO₂.
 10. The method of claim 8,wherein said first etchant and said second etchant each comprisephosphoric acid.
 11. The method of claim 8, wherein said predeterminedthreshold is about 50:1.
 12. The method of claim 8, wherein saidselective etchant comprises between about 1% and about 30% of said firstetchant by volume.
 13. A method for exchanging an etching bath solution,comprising: providing an etching system comprising an etching bathhaving a first etchant disposed therein, a receiving tank connected tosaid etching bath and configured to collect said first etchanttransferred from said bath; etching Si₃N₄ and SiO₂ from at least onesubstrate into said first etchant, said etching comprising dissolvingsaid SiO₂ and said Si₃N₄ in said first etchant; determining that anetching rate ratio of Si₃N₄ to SiO₂ from said at least one substrate insaid first etchant is below a threshold value; in response to saiddetermining, transferring said first etchant from said bath to saidreceiving tank; after said transferring said first etchant, adding asecond etchant to said bath; and transferring a portion of said firstetchant from said receiving tank to said bath, said portion mixing withsaid second etchant in said bath, said first etchant having said Si₃N₄and said SiO₂ dissolved therein.
 14. The method of claim 13, whereinsaid first acidic etchant and said second liquid acidic etchant eachcomprise phosphoric acid.
 15. The method of claim 14, wherein said firstacidic etchant is at a temperature between about 100° C. and about 165°C.
 16. The method of claim 13, wherein said etching ratio is less thanabout 50:1.
 17. The method of claim 13, wherein said portion mixing withsaid second etchant comprises forming a final volume of etchant, whereinsaid portion is between about 1% and about 30% of said final volume ofetchant.
 18. The method of claim 13, wherein said Si₃N₄ has an etch rateless than about 50 angstroms/minute.
 19. The method of claim 13, whereinsaid SiO₂ has an etch rate greater than about 1 angstrom/minute.
 20. Themethod of claim 13, wherein said first etchant and said second etchanteach have an acid concentration greater than about 85% by weight.